The present invention relates to a level converter circuit and, more particularly, to such a circuit that converts an input signal having a small amplitude into an output signal having an ECL (emitter-coupled-logic) level.
At present, there are various types of logic gate circuits such as TTL's, ECL's, CMOS's and so on. It is therefore required to provide a level converter circuit in order to transferred a signal produced by one logic gate circuits to another logic gate circuits. For example, in a case of where a signal to be processed is supplied to an ECL circuit, such a circuit is required that converts a signal into an ECL signal.
Referring to FIG. 1, there is shown an ECL converter circuit according to the prior art. This circuit includes a differential amplifier 30 consisting of NPN transistors 31 and 32, a current source 311, and resistors 37 and 38, which are connected as shown. The bases of the transistors 31 and 32 are connected respectively to input terminals VIN1 and VIN2 to which true and complementary input signals are supplied. This circuit operates on a positive power supply voltage VCC and a negative power supply voltage VEE.
The output of the amplifier 30 is derived from the collector of the transistor 32 and supplied to a level shift circuit 40. This circuit 40 includes an NPN transistor 33, diodes 312, 313 and 314, a resistor 39, and an NPN transistor 34. The collector of the transistor 34 is grounded and the emitter voltage thereof is connected to an output terminal VOUT from which a level-converted signal is derived.
Assuming the base-emitter voltages of the transistors 33 and 34 are VBE33 and VBE34, respectively, the voltages drop across each of the diodes 312, 313, and 314 is VD3, and the voltage across the resistor 38 be called VR38, the voltage V0 of the output or level-converted signal is presented as follows: EQU V0=VCC-VR38-VBE33-3VD3-VBE34. (1)
As is apparent from the equation (1), the level V0 of the output signal is dependent on the power supply voltage VCC. For this reason, the variation in the power supply voltage VCC causes the level of the output signal to vary.